A copolymer of styrene and maleic anhydride (SMA) was dissolved in dimethyl sulphoxide (DMSO) and injected into the lumen of the vas deferens of rats. The polymer was retained in the lumen for a period of more than 90 days.Sodium bicarbonate (10%), pH 8.9, was used to flush the polymeric material from the vas deferens lumen. The reversibility and patency of the vas deferens was thus obtained.Sodium bicarbonate proved to be an effective alternative to dimethyl sulphoxide for reversal.